1. Field of the Invention
The present invention relates to an imaging device and, more particularly, to a solid-state imaging device formed using a semiconductor device.
2. Description of the Related Art
The solid-state imaging device has various advantages over the image pick-up tube, for example, a smaller size, lighter weight, maintenance-free properties and a longer lifetime. As a result of remarkable progress in semiconductor technology, the solid-state imaging device is replacing the image pick-up tube because of its superiority over the latter in terms of production cost and performance.
Such solid-state imaging devices may roughly be classified into two types, that is, the MOS type in which the photo-electric charges are generated in each photo-electric conversion elements (photo-diodes) and are externally read out through an aluminum signal line via MOS transistors, and the CCD type in which the photo-electric charge generated in each photo-electric conversion elements are externally read out by means of CCD (Charge Coupled Device) shift registers.
CCD type solid-state imaging devices include: the frame transfer type in which the imaging area and the storage area are provided independently of each other; the interline type which has stripe-shaped storage and transfer areas; and the FIT (Frame Interline Transfer) type which is a mixture of said two types. Since these CCD type solid-state imaging devices are capable of highly efficiently outputting the photo-electric charges generated in each photo-electric conversion element to the outside, fewer noises are generated therein than in the MOS type solid-state imaging devices, advantageously.
Such a CCD type solid-state imaging device is described in ISSCC 82, p. 168 to 169 and in ISSCC 86, p. 94.
A conventional solid-state imaging device will be described hereinunder with reference to FIGS. 1 and 2. FIG. 1 shows the arrangement of a CCD type solid-state imaging device, and FIG. 2 is a sectional view taken along the line A--A' of FIG. 1.
Each of the photo-electric conversion elements 1 is a photo-diode which is defined by, for example, a pn junction diode, and adapted to convert incident light to an optical signal charge corresponding to the intensity of the incident light and to store the charge. Each of the vertical CCD registers 3-1 and 3-2 is provided for each of the photo-electric conversion element rows (1-1, 1-2, 1-3) and (1-4, 1-5, 1-6) arranged so as to extend vertically as viewed in FIG. 1, the vertical CCD registers being adapted to transfer charges in the vertical direction. Each of the selector gates 2 is provided between the corresponding photo-electric conversion element 1 and vertical CCD register 3 to control the flow of the signal charge from the photo-electric conversion element 1 to the vertical CCD register 3. A horizontal CCD register 11 is provided at either side of the group of vertical CCD registers 3 to receive in parallel signal charges which are respectively transferred through the vertical CCD registers 3 and to transfer the received signal charges in the horizontal direction. An output amplifier 12 is provided at one end of the horizontal CCD register 11 to amplify and output the signal charges transferred through the horizontal CCD register 11.
The optical signal charge stored in each photo-diode 1 is transferred to the corresponding CCD register 3 through the associated selector gate 2. The transferred signal charge is further transferred to the output amplifier 12 through the vertical CCD register 3 and the horizontal CCD register 11 and finally outputted from the output amplifier 12 in the form of an imaging signal.
In general, the vertical CCD registers 3 are driven by 4-phase clocks. Although not shown, 4-phase clock wirings are provided for each vertical CCD register 3, and among the 4-phase clock wirings, those for 2-phases also serve as gate wirings for the selector gates 2.
FIG. 2 is a sectional view taken along the line A--A' of FIG. 1, which shows an imaging area consisting of the photo-diodes 1-3, 1-6, the selector gates 2-3, 2-6 and the vertical CCD registers 3-1, 3-2.
A semiconductor substrate 6 of a first conductivity type (n-type) is formed therein with an impurity layer (well) 13 of a second conductivity type (p-type). An n.sup.+ -type impurity layer is formed within the p-type impurity layer 13, the n.sup.+ -type impurity layer being used to constitute the photo-diodes 1-3, 1-6 and respective channel regions 3'-1, 3'-2 of the vertical CCD registers 3. The p-type impurity layer 13 and the n.sup.+ -type impurity layer constitute in combination the pn junction photo-diodes 1. An optical light charge which is generated in response to light entering the imaging area is stored in the n.sup.+ -type impurity layer. The photo-diodes 1-3, 1-6 and the channel regions 3'-1, 3'-2 of the vertical CCD registers 3 form source and drain regions of MOS (Metal Oxide Semiconductor) transistors having their gates defined by the selector gates 2-3 and 2-6, respectively. The selector gates 2-3 and 2-6 also serve as transfer electrodes of the vertical CCD registers 3. Insulators 7-1 and 7-2 formed from, the example, SiO.sub.2, are insulators for isolation between the photo-diodes 1-3, 1-6 and the channel regions 3'-1, 3'-2 of the vertical CCD registers 3. Gates 8-1 and 8-2 serve as transfer electrodes of the vertical CCD registers 3. Light-shielding films 9-1 and 9-2 made of, for example, aluminum, prevent light from entering the vertical CCD registers 3. An insulator 10 is an insulating film between up and down layers which is made of, for example, SiO.sub.2, to isolate adjacent gates from each other.
The solid-state imaging device of the type described above suffers, however, from the following problems. Among rays of light entering the device, some of the components of light which are obliquely incident thereon and the components of light which are reflected from the interface between the SiO.sub.2 10 and the Si substrate 6 may leak in regions within the p-type impurity layer 13 which are in the vicinities of the channel regions 3'-1 and 3'-2 of the vertical CCD registers 3. A part of noise charge which is generated in response to such leakage light may further enter the channel regions 3'-1 and 3'-2 of the vertical CCD registers 3 by the action of diffusion or an electric drift which causes the smear phenomenon that makes the image blurry and unclear.